Low Biogas Yield Research Articles

Techno-Economic Assessment of Anaerobic Co-digestion of Cattle Manure and Wheat Straw (Raw and Pre-treated) at Small to Medium Dairy Cattle Farms

Anaerobic digestion of dairy cattle manure is a common practice; however, the low biogas yield of manure can hamper the profitability of anaerobic digestion systems in small to medium dairy cattle farms . To make this technology more attractive to farmers, an increase in biogas yield per cubic meter of reactor could be achieved by co-digesting animal manure with co-substrates such as straw (in its raw form or pre-treated). A Techno-economic assessment of anaerobic co-digestion of cattle manure and raw and briquetted straw has been carried out. The results obtained from the techno-economic assessment showed that for a farm of 250 adult cattle heads the revenues generated in an anaerobic mono-digestion process are not able to offset the initial required investment. However, the co-digestion of manure with raw or briquetted straw showed better economic performance and positive returns (net present values > 0, internal rate of return > 9% and a RETURN of the investment in 11 years) for farmers willing to implement anaerobic digestion. Electricity selling price, and the price of the straw are the key parameters to determine the profitability of the system. Moreover, pre-treatments to increase the straw biogas production have been assessed and evaluated from a technic and economic perspective. Alkali and microwave-alkali straw pre-treatments showed the best results with an increase in biogas production of 156% and 92% compared to raw straw.

A review on enhanced biogas production from anaerobic digestion of lignocellulosic biomass by different enhancement techniques

Abstract Anaerobic digestion of lignocellulosic biomass has gained huge attention in current years, especially for degrading agricultural waste (i.e. rice straw, wheat straw etc.). Challenges related to anaerobic digestion of agricultural waste are low biogas yield, poor buffering capacity, low quality end products and potential variability. These inadequacies are either because of low mass transfer (high solid contents, poor nutritional structure) or can be accredited to recalcitrant nature of agricultural residues. To overcome these challenges various techniques exists like pretreatment of feedstock, co-digestion with different substrate and inoculation efficiency have been considered to increase the performance of anaerobic process. This review includes the effect of these techniques on lignocellulosic degradation, synergistic interaction with high C/N ratio feedstock and improving the buffering capacity by inoculum ratio optimization. Furthermore, key parameters influencing the anaerobic digestion are included with proposed future study.

Anaerobic digestion and the effect of hydrothermal pretreatment on the biogas yield of cocoa pods residues

The current research investigated the possibility of valorizing cocoa pods residues through anaerobic digestion and the possibility of increasing the biogas yield by hydrothermal pretreatment. Using a central composite surface response methodology, the effect of temperature and reaction time on the hydrothermal pretreatment process was studied. Temperature and reaction time was varied between 150–220 °C and 5–15 min respectively. The result show that untreated cocoa pods residues has a biogas potential of 357 l(N)/kgVS and a methane content of 55%. The effect of hydrothermal pretreatment on the biogas yield was diverse. Severities below 3.0 resulted in increased biogas yield. However, higher severity resulted in lower biogas yield. The optimum biogas yield (526.38 l(N)/kgVS) was obtained at 150 °C and 15 min, which represents a severity of 2.65.

Simultaneous biogas and biogas slurry production from co-digestion of pig manure and corn straw: Performance optimization and microbial community shift

Anaerobic co-digestion (AcoD) is proved as an effective approach to solving a bottleneck problem of the low biogas yield in agricultural biomass waste treatment with anaerobic digestion (AD) technology. The present study investigated the effect of C/N radio, organic loading rate (OLR) and total solids (TS) contents on reactor performance in AcoD of pig manure and corn straw for simultaneous biogas and biogas slurry production. It was found that the highest biogas production was obtained at C/N ratio of 25, while the best biogas slurry performance was achieved at C/N ratio of 35. And high OLR and TS resulted in good performances in both biogas production and biogas slurry. At last, the microbial community analysis suggested that Bacteroidetes played a significant role in AcoD process. Acetoclastic methanogenesis was the main pathway for methane production in the stable system. And changing operational parameters could transform and shift the microbial community.

Effects of pre-treatments and co-digestion on biogas production from Okra waste

The effects of pre-treatment and co-digestion on the biogas yield of okra wastes are the focus of this paper. Okra wastes (O) were treated with 5% alkaline (NaOH) (OA), microwave (OMW), and microwave + alkaline. Sheep slurry (S) was co-digested with okra waste in varying proportions [25% (OS25), 50% (OS50), and 100% (S)]. Biomethane potential tests using 500 ml bioreactors were carried out in the batch mode at a hydraulic retention time of 25 days under mesophilic conditions (37 °C). The cumulative biogas yield from the alkaline treated sample (OA) increased by 45.87% when compared to the control (O), and other pre-treated samples were statistically the same with untreated okra waste at the 95% confidence level. Samples co-digested with sheep slurry recorded a low biogas yield. The results of the kinetic studies performed with the measured cumulative biogas yield showed that while the modified Gompertz model presented a better prediction range (0.66%–2.48%), the first-order kinetic model produced a better goodness of fit (<10) and R2 values when compared to the modified Gompertz model. The results of the Artificial neural network (ANN) performed were compared with the earlier used kinetic models, and it was observed that all the performance indicators (R2, RMSE and %prediction difference) from the ANN model best predicted the biomethane yields.

FISH analysis of microbial communities in a full-scale technology for biogas production.

The anaerobic digestion is a biological process that consists of four stages. At the final step of the biodegradation of the organics the most sensitive to the ambient factors group of microorganisms - the methanogens, produces biogas with main component methane. Common problems of these technologies are low biogas yield, production of biogas with low quality or situations in which the plant gets out of exploitation. These problems are related to the lack of biological indicators of the process used in the practice and lack of understanding of the structure and functioning of the methanogenic consortium. Different fluorescent techniques have the potential to fulfill this gap and to contribute to the deep understanding of the structure of the microbial communities. In this study it was applied fluorescence in situ hybridization analysis for identifying and localization of microorganisms by the Archaea domain in digesters of wastewater treatment plant "Kubratovo". High negative correlation between the quantity of Archaea and the biogas and methane production has been registered. This method has the potential to be used as a tool for analyzing the structure of the microbial communities in the digesters and thus to allow the adaptation of the consortium and the optimization of the whole process.

Forage types and origin of manure in codigestion affect methane yield and microbial community structure

AbstractIn farm‐scale biogas systems, different kinds of manure are the most important substrate for anaerobic digestion, but result in low biogas yields. Biogas production can be increased by complementing the manure with forage crops, in codigestion. The aim of this study was to evaluate grass‐clover (GCS) and whole‐crop barley silages (WCB) in codigestion with manure from organic and conventional dairy production systems on biogas production, microbial community, degree of degradation and gas quality at different organic loading rates by addition of soya bean meal and wheat grain, which are rich in protein and starch. Four continuous stirred anaerobic laboratory‐scale reactors were used, and the codigestion resulted in additive effects on biogas production, but no synergistic effects. The highest biogas yield was obtained in reactors receiving WCB independently of manure types, for both experiments (7,416 ml/day and 10,978 ml/day respectively). The degradation efficiency, measured as the reduction in volatile solids was, on average, six percentage units higher in the reactors receiving manure from conventional compared with organic dairy cows, probably because of a higher concentration of undigested fibre and proteins in conventional cow manure. Microbiological analysis by illumina sequencing illustrated low impact of both manure types on the reactor community and only small differences between the reactors receiving GCS and WCB. However, addition of soya bean meal and wheat grain changed the community in all reactors. The ratio between Firmicutes and Bacteroidetes was comparably higher in reactors having the highest gas production and methane yield.

Effect of ultrasonic pretreatment on the semicontinuous anaerobic digestion of waste activated sludge with increasing loading rates

Anaerobic digestion (AD) is the most widely used method to stabilize and recover energy from waste activated sludge (WAS). However, AD of sludge results in a low biogas yield. Ultrasonic pretreatment (USp) is the most effective sludge disintegration technology and has elicited interest to increase the biodegradability and improve the efficiency of the AD. This study investigated the influence of USp on the semicontinuous AD of WAS as a strategy to increase its performance. During an experiment lasting 90 days, 0.8-L digesters were semicontinuously fed under different conditions: raw sludge and sonicated at 15000, 25000 and 35000 kJ/kg TS (total solids); and an increase of the organic loading rate (OLR) from 1 to 3 kgVS/m3·d, for 30 days each. The maximum biogas rate (0.218 L/d) was reached at 25000 kJ/kg TS and 3 kgVS/m3·d. The highest VS removal efficiency (23.7%) was observed at the highest USp and the lowest OLR. The reactors fed with raw sludge had a lower performance compared to the sonicated WAS in terms of VS reduction and biogas production. In contrast, the USp allowed the bioreactors to be operated at a high OLR (3 kgVS/m3·d) without inhibition. Finally, a kinetic analysis of accumulated biogas was performed.

Influence of sheep manure addition on biogas potential and methanogenic communities during cow dung digestion under mesophilic conditions

Abstract The efficient treatment of animal slurries can support the bioenergy management and environmental protection; however, the low biogas yield and quality are the major constraints. The object of this paper is to investigate how the co-digestion of sheep manure and cow dung by not using inoculum influences the performance of the process and determine the methanogenic communities at the end of the experiment. Biochemical Methane Potential essays were conducted in mesophilic conditions in order to determine the biogas-methane potential. Enhanced biogas production was achieved from the mono-digestion of cow dung with 104.3 NmL biogas g−1 VS and the co-digestion of cow dung and sterilized sheep manure with a lower biogas yield of 89.0 NmL biogas g−1 VS.

Experimental and feasibility assessment of biogas production by anaerobic digestion of fruit and vegetable waste from Joburg Market

Substrate-induced instability of anaerobic digestion from fruit and vegetable waste (FVW) results in low biogas yield. In this study, substrate management through fruit to vegetable mix ratio in a two-stage semi-continuous digester was investigated as a pathway for optimality of yield. The experiment conducted over 105 days with 62.52 kg of FVWs sourced from Joburg Market, South Africa showed that a stable process was achieved at a fruit to vegetable waste mix ratio of 2.2:2.8. At this ratio, optimal organic loading rate ranged between 2.68 and 2.97 kg VS/m3-d which resulted in a specific biogas yield of 0.87 Nm3/kg VS with 57.58% methane on average. The results of the experimental study were used as a feasibility assessment for a full-scale 45 tonnes/d plant for Joburg Market considering three energy pathways. The plant will produce 1,605,455 Nm3/y of biogas with the potential for offsetting 15.2% of the Joburg Market energy demand. Conversion of all biogas to biomethane was the most economically attractive energy pathway with a net present value of $2,428,021, an internal rate of return of 16.90% and a simple payback period of 6.17 years. This route avoided the greenhouse gas emission of 12,393 tonnes CO2, eq. The study shows that the anaerobic digestion of FVWs as sole substrate is possible with financial and environmental attractiveness.

Mechanistic investigations in ultrasonic pretreatment and anaerobic digestion of landfill leachates

This study has attempted to gain mechanistic insight into ultrasound assisted pretreatment of landfill leachate (prior to anaerobic digestion) by coupling experimental results with simulations of cavitation bubble dynamics. Essentially, individual roles of physical and chemical effects induced by sonication in the process have been identified. Changes in properties of leachate (such as sCOD, tCOD, NH3-N and acetate concentrations) subjected to sonication at different power levels have been studied. Sonication generated intense microconvection in system that caused faster disintegration and solubilization of biosolids. However, for high power input, where transient cavitation occurred in solution, degradation of organic matter by oxidizing radicals generated by cavitation bubbles occurred that reduced sCOD value resulting in low biogas yield in anaerobic digestion. Sonication at low to moderate power input, where stable cavitation prevailed, caused disintegration and solubilization, but not degradation, of biosolids resulting in higher sCOD value and biogas yield.

Evaluating the effect of enzymatic pretreatment on the anaerobic digestibility of pulp and paper biosludge

Anaerobic digestion of biosludge has not yet been implemented in pulp mills due to low biogas yields. Enzymatic pretreatment of biosludge has shown improvements in biogas yields but results are varied. A key limitation of previous studies is that they fail to consider the COD contribution from the enzyme solutions. The aim of this study was to systematically investigate the potential for enzymatic pretreatment on the anaerobic digestibility of pulp mill biosludge. Out of the six enzymes tested, four enhanced the anaerobic digestibility of biosludge. At the end of the BMP, a maximum improvement of 26% in biogas yield was observed with protease from B. licheniformis. There was no correlation between enzymatic activities on standard substrates and/or on biosludge and the effect of enzymes on biogas yields. Enzymes have potential for improving biosludge anaerobic digestibility but more research on optimal conditions and potential synergies with other pretreatment is needed.

Evaluation of Biogas Production Potential from Raw and Processed Agricultural Wastes

In this study, the evaluation of biogas production potential from 5 raw and processed agricultural wastes: soybean residues, papaya peels, sugarcane bagasses, rice straws and greater galangals were investigated. The experimental study was carried out into 2 parts. Firstly, the batch experiment was conducted in mesophilic conditions (30°C), at five different hydraulic retention times (HRT): 15 days, 20 days 25 days 30 days and 35 days. The results revealed that soybean residue generated the highest biogas yield of 560.47 ml. at the HRT of 25 days. Papaya peels produced the highest biogas yield of 404.24 ml. at the HRT of 15 days. Bagasse generated the highest biogas yield of 263.04 ml. at the HRT of 25 days. Straw and greater galangal showed a very low biogas yield under the studied conditions. Secondly, the continuous experiment was carried out under an optimal HRT of each feedstock using a digester of 200 liters between 60 days. The average biogas production rate of soybean residue, papaya peel, bagasse, straw and greater galangal was 63.01, 54.63, 16.28, 13.94 and 0.68 L/days, respectively. Finally, the methane yield of biogas produced from different feedstocks was analyzed using a gas chromatography. The results shown that biogas from soybean residue, papaya peel, bagasse, straw and greater galangal precedes the methane yield of 57.14%, 53.70%, 49.12%, 56.25% and 73.50%, respectively.

Improvement of Solid-State Anaerobic Digestion of Yard Waste by Co-digestion and pH Adjustment

To make clear the feasibility of controlled anaerobic digestion (AD) of organic fraction of municipal solid waste being implemented at transfer stations prior to their final disposal, the performance of solid-state AD (SSAD) of yard waste (YW) as well as the improvement by its co-digestion with food waste (FW) in combination with the aid of pH adjustment were investigated by conducting batch SSAD experiments. The results showed that AD of YW with high solid contents easily caused difficulties in the initiation stage due to accumulation of volatile fatty acids. Even with the aid of alkali to successfully initiate the initiation stage, the AD of YW with the high solid content of 29% generated such a low biogas yield as 207 mL/g VS. By contrast, co-digestion of YW and FW with pH adjustment gave the highest biogas and methane yields of 306 and 180 mL/g VS, 47.8 and 40.3% higher than those for AD of YW alone, respectively. A sufficiently long acidogenesis duration was helpful for biogas production, while the inhibitions from excessively high ammonia nitrogen concentrations and organic loading rates were apt to happen after extremely long acidogenesis durations. For co-digestion of YW and FW, adding proper amounts of alkali (CaO) on the 3rd days resluted in the best performance.

Anaerobic Co-digestion of Swine Manure and Microalgae Chlorella sp.: Experimental Studies and Energy Analysis

Integration of algae production with livestock waste management has the potential to recover energy and nutrients from animal manure, while reducing discharges of organic matter, pathogens, and nutrients to the environment. In this study, microalgae Chlorella sp. were grown on centrate from anaerobically digested swine manure. The algae were harvested for mesophilic anaerobic digestion (AD) with swine manure for bioenergy production. Low biogas yields were observed in batch AD studies with algae alone, or when algae were co-digested with swine manure at ≥43 % algae (based on volatile solids [VS]). However, co-digestion of 6–16 % algae with swine manure produced similar biogas yields as digestion of swine manure alone. An average methane yield of 190 mL/g VSfed was achieved in long-term semi-continuous co-digestion studies with 10 ± 3 % algae with swine manure. Data from the experimental studies were used in an energy analysis assuming the process was scaled up to a concentrated animal feeding operation (CAFO) with 7000 pigs with integrated algae-based treatment of centrate and co-digestion of manure and the harvested algae. The average net energy production for the system was estimated at 1027 kWh per day. A mass balance indicated that 58 % of nitrogen (N) and 98 % of phosphorus (P) in the system were removed in the biosolids. A major advantage of the proposed process is the reduction in nutrient discharges compared with AD of swine waste without algae production.

Dry co-digestion of sewage sludge and rice straw under mesophilic and thermophilic anaerobic conditions.

Dry anaerobic digestion of sewage sludge can recover biogas as energy; however, its low C/N ratio limits it as a single substrate in the anaerobic digestion. Rice straw is an abundant agricultural residue in China, which is rich in carbon and can be used as carbon source. In the present study, the performance of dry co-digestion of sewage sludge and rice straw was investigated under mesophilic (35 °C) and thermophilic (55 °C) conditions. The operational factors impacting dry co-digestion of sewage sludge and rice straw such as C/N ratio, moisture content, and initial pH were explored under mesophilic conditions. The results show that low C/N ratios resulted in a higher biogas production rate, but a lower specific biogas yield; low moisture content of 65 % resulted in the instability of the digestion system and a low specific biogas yield. Initial pH ranging 7.0-9.0 did not affect the performance of the anaerobic digestion. The C/N ratio of 26-29:1, moisture content of 70-80 %, and pH 7.0-9.0 resulted in good performance in the dry mesophilic co-digestion of sewage sludge and rice straw. As compared with mesophilic digestion, thermophilic co-digestion of sewage sludge and rice straw significantly enhanced the degradation efficiency of the substrates and the specific biogas yield (p < 0.05) at the conditions of C/N ratio 26:1, moisture content 80 %, and natural initial pH. Although high concentrations of ammonia-nitrogen (NH4-N, 1500 mg/kg wet weight) were formed during thermophilic digestion, there was no obvious inhibition occurred. The results indicated that rice straw can be used as carbon source for the dry co-digestion of sewage sludge under mesophilic and thermophilic conditions.

Physicochemical changes of rice straw after lime pretreatment and mesophilic dry digestion

A better understanding of the physicochemical properties of the material, its changes and the effects of anaerobic digestion, will help to improve methane production efficiency. The physicochemical changes of rice straw after solid-state lime pretreatment and dry digestion were investigated. Those changes were revealed by the composition of the substrate before any treatment, from samples of the material during the process and by the final product. Such compositions were studied with X-ray, Fourier transform infrared spectroscopy and thermogravimetric analyzes. The obtained results showed that low mass transfer limited the lignin modification and polysaccharide decomposition, resulted in a low biogas yield; and that after pretreatment and 60 days of mesophilic dry digestion, the concentration of cellulose decreased (23.4%) as well as hemicellulose (13.1%). In the opposite case, the concentration of lignin increased (59%) as well as ash (108%). The crystallinity index increased by 5.4% after pretreatment and decreased by 4.8% after digestion. The FTIR spectrogram of the raw sample included a peak reflecting absorption of ester linkages. This peak was absent from FTIR spectra of pretreated and digested samples, indicating that such linkages are broken during pretreatment and then digested by the microorganisms during the anaerobic fermentation. Furthermore, lime pretreatment and dry digestion significantly changed the decomposed profiles and rate, the distribution of pyrolysis product. The obtained data showed that polysaccharides and lignin were decomposed during pretreatment and digestion phases, leading to changes in structural and thermal properties.

IMPROVING SPECIFIC POWER CONSUMPTION FOR MECHANICAL MIXING OF THE FEEDSTOCK IN A BIOGAS FERMENTER BY MECHANICAL DISINTEGRATION OF LIGNOCELLULOSE BIOMASS

&lt;p&gt;Lignocellulosic biomass particles in biogas fermenter batch either sediment towards vessel bottom or rise towards batch surface, where they float and form a compact thick scum. These processes have primarily the negative influence on batch homogeneity, on evenness of batch temperature field, on removal of produced biogas bubbles out of liquid batch and also on mass transfer among microorganisms. These facts result in non-effective usage of biomass energy-potential that entails in low biogas yields. Therefore, good mixing of bioreactor batch is very important in order to stabilize anaerobic digestion process. The aims of the present study were to evaluate the impact of wheat straw disintegration and its hydration on hydrodynamic behaviour and on specific power consumption for mechanical mixing of wheat straw-water suspension. Based on experimental results, it was concluded that both hydration and mechanical disintegration of lignocellulosic biomass significantly improve homogeneity and pump-ability of biomass-water batches. Wheat straw hydration itself decreases specific power consumption for batch mixing by 60 % towards untreated straw. Moreover, mechanical disintegration itself decreases specific power consumption by 50 % at least towards untreated hydrated straw.&lt;/p&gt;

Optimization of the digestion process of Scenedesmus sp. and Opuntia maxima for biogas production

Scenedesmus biomass is not an adequate substrate for anaerobic digestion due to its low biodegradability and low biogas yield. This study aims to evaluate the anaerobic co-digestion of Scenedesmus microalgal biomass and Opuntia maxima cladodes, the latter added in order to improve the digestion process. Batch assays were conducted to evaluate possible synergistic effects in different mixtures of both substrates. Mixture with highest methane yield was digested in semi-continuous mode at different VS concentrations. Feedstock composed of 75% O.maxima and 25% Scenedesmus (VS basis) showed the highest methane yield increasing 66.4% and 63.9% that of Scenedesmus and O.maxima, respectively. In semi-continuous mode, ideal organic loading rate (OLR) with 6%VS feed concentration was 4gVSL−1d−1, which yielded 292±39LCH4kgVS−1 (15days HRT). In the case of 8%VS feed concentration ideal OLR was 5.33gVSL−1d−1, which yielded 308±22LCH4kgVS−1 (15days HRT). The co-digestion of O.maxima and Scenedesmus biomass enhanced the anaerobic digestion process and avoided inhibition caused by low C/N ratio of microalgae.

Fungi open new possibilities for anaerobic fermentation of organic residues

Large amounts of fibre-rich organic waste material from public green and private gardens have to be treated environmentally friendly; however, this fibre-rich biomass has low biogas yields. This study investigated the presence of fungi in full-scale biogas plants as well as in laboratory reactors and elucidated the importance of fungi for the biogas process. The dominating members of the eukaryotic community were identified by analyzing 18S rRNA gene and internal transcribed spacer 1 (ITS1) region fragments of clone libraries. These identifications were accompanied by diverse microscopic techniques such as fluorescence microscopy and conventional scanning electron microscopy. Cells of presumably fungal origin were characterized by intensive fluorescence and were about 1 order of magnitude larger than prokaryotic cells. Molecular techniques enabled to identify fungi from the subphyla Agaricomycotina, Mucoromycotina, Pezizomycotina, Pucciniomycotina and Saccharomycotina and from the class Neocallimastigomycetes. Members of these groups can be important for microbial degradation of complex compounds, due to the ability to penetrate cell walls, and thus open the cells for the influx of bacteria, further enhancing degradation. Optimal treatment of biowaste depends on the amount of lignocelluloses. Targeted application of fungi to the biogas process will open wider possibilities for anaerobic treatment of fibre-rich biomass and can result in better biomass utilization as a renewable energy resource. Due to higher temperature optima of fungal cellulolytic enzymes, the thermophilic process is suggested for anaerobic degradation of fibre-rich biomass.